Iron-aluminum materials for magnetic applications



2,875,114 IRON-ALUMINIM MATERIALS FOR MAGNETIC APRLICATIONS Paul A. Albert, Pittsburgh, Pa., asslgnor to Westinghouse Electric Corporation, tion of Pennsylvania Drawing. Application April 12, 1957 Serial No. 652,372

8 Claims. (Cl. 148-120) This invention relates to materials to be used for particularly relates to new to the development of a new class of materials that are useful, in general, for purposes similar to theuses of commercial magnetic iron-silicon alloys and iron-nickel alloys.

The object of this invention is to provide for aluminumiron alloy sheets having improved magnetic properties, by a double or triple cold rolling process following by intervening anneals and a final anneal under specified conditions, whereby sheets having a double oriented cube on face grain structure are produced. It is another object of this invention to provide materials that may be used for magnetic applications, which are made only of metals that have a low strategic index. Still another object of the invention is to provide processes for producing iron-aluminum'materials according to the foregoing objects that may readily be practiced with commercially available facilities and that are adaptable inprocedure to variations whereby materials be obtained. a a H I have discovered, and is on this discovery that the invention is in largepart predicated, that upon subjecting ductile iron-aluminum alloysto a series of cold rolling with properties tailored for particular applications, may,

andheat treating operations to produce thin sheets in which the g rains are oriented with, a cu be plane, and thus two easy directions of magnetism, in the sheet SUI", face, andwhich exhibits outstanding magnetic properties.

I havealsofdiscoveredthat control of particular steps,

in the series of operations may be used to develop particular properties to values over and above those generally obtained not I have been able to produce, by

way of example, iron-aluminum tapes for magnetic applications that exhibit high saturation induction and high induction atlow East Pittsburgh, Pa., a corporaprepared from iron-aluminum alloys,

in the resultantmaterial. In this man United States Patent fieldsand generally are particularly useful for various magnetic applications.

The iron-aluminum products of this invention are obtained as flat sheets, stn'psor tapes of athickness on the order of r 1 to 18 rnils. Other shapes may be pro, duced for particular applications by use of shearing or stamping techniques.

tions E or L shapes may be stamped or punched from sheets ortapes. Broadly these products are characterized by an induction at afield of 10 oersteds of atleast 12,000ugauss, a coercive force of at least asjlow as 0.5 oersted, and a pulse permeability at afield'of 1O oersteds of at :least700. From the data presented hereinafter it will: be apparent'that thevalues of the for'egoing properties in tapes of this invention are generally improved For example, for specialapplica "ice over the indicated levels when the preferred practices are carried out. By way of example, for low remanence tapes of 1 to 6 mils, which are of particular interest for pulse-permeability material, the remanence usually is at least as low as 5,500 gauss, the induction at a field of 10 oersteds is at least 13,500 gauss, and the coercive force is at least as low as 0.5 oersted. Similarly, for tapes of 1 to 6 mils that have been prepared with a view to high inductions the properties are an'induction at a field of 10 oersteds of at least 14,500 gauss and a coercive force not exceeding 0.45 oersted. For these latter materials where remanenceis not a controlling property, the remanence is on the order of 9,500 gauss.

The products of this invention are obtained by a process involving a plurality of cold reductions of an ironaluminum alloy plate, or strip, suitably with intermediate heat treatments and a final heat treatment. Three cold rolling operations normally are used. In the first cold roll, a reduction of about 60'to percent is taken. A reduction of about 80 to 97 percent of the then thickness is taken in the second cold roll, while a further reduction equivalent to about 40 to percent of the thickness of the strip from the second cold roll is taken in the Within these general limits particular third cold roll. combinations of reductionsmay be adopted with a view to the development of characteristics in the final product that may be desired.

After each cold roll the sheet is given an annealing heat treatment under temperature and time conditions to secure certain desired recrystallization structures, as well as to produce stress relief, in a non-oxidizing or reducing atmosphere. More particularly, after the first cold roll the sheet is given a primary recrystallization anneal which comprises heating the sheet to a temperature,of from 850 C. to 1100 C. for a period of from about two to three hours at the lower temperaturesand about 15 minutes at the highest temperatures. After the second cold roll the sheet is first annealed in the primary recrystallization range, i. e. above 600 C. and suitably at 700 to 850 C., for a few minutes up to about two hours, and then placed in a furnace which is at atemperature sufficient to result in the large grain growth characteristic, of secondary recrystallization, i. e. at about1050 to l300 C. for one-half to twelve hours.

I have found that the heat treatment applied after the third cold roll is particularly important with respect to the magnetic properties of the treatment after the third cold roll includes, normally, an anneal in the primary recrystallization temperature range, that is at 700 to 850 C. or slightly higher for one to ten hours, followed by slowly heating the sheet to a higher temperature and heat-treating the sheet at that higher temperature until the desired properties result. The heating rate. may vary from about 5 to Caper hour, and the heat-treating temperature is in the range of, 900 to ,1250 C. and is maintained for one to, twelve hours. invention limited by theory, it is believed that double, or cube-on-face, orientation maximizes during the heat treatment after the third cold roll. By double orientation I mean to indicate an oriented grain structure having an easy direction of magnetismalong and perpendicularto the rolling direction. Within the foregoing ranges, it will be apparent that variations can be made with a view to developing special characteristics. For example, selected heating after the third cold roll has been found desirable with respect to obtaining a low remanence. Consequently, where that property is most important, the sheets are heated at 1000? C. and higher.

As will be evident to thosej familiar with the art of strip or tape should be provided at such times as the end product. p The heat While it is not intended to have the -2,sr5,114 V a article is at a temperature; such as above' about 600 C.,, at which deleterious oxidation may occur, By protective atmosphere I intend to indicate any suitable nonoxidizing atmosphere such, for example, as argon or y roge Iron-aluminum alloys that may be used in practicing the. present invention are ductile iron'aluminum alloys. Such alloys generally contain aluminum in an amount of; about 3 to 7 percent based on the weight of the'alloy. Other alloying constituents and incidental impurities-may be present in varying amounts providing that they do not deleteriously interfere with obtaining the described properties in the end products. These alloys conventionallyare produced by melting, in a vacuum furnace, a pure iron, such as electrolyticiron, and an aluminum suitably having a purity on the order of 99.00 percent. The melt may be cast in vacuum or under a protective atmosphere such, for example, as argon, to prevent the uncontrolled introduction of impuritiesfrom the atmos- Ph eture above the recrystallization temperature to. a thickness of about 0.4 to 0.65 inch, suitably about 0.5 inch,

prior to the first cold rolling step above mentioned.

The following examples, demonstrating the production of tapes for magnetic applications, will serve to illustrate.

the invention. These examples and the details thereof are given by way of tion.

EXAMPLE I An alloy ingot'is prepared by melting commercially available electrolytic iron and high-purity aluminum, in a weight ratio of 94.5 parts of iron to 5.5 parts of the aluminum, in an induction furnace under argon at atmospheric pressure, and then casting the melt. The

resulting ingot is hot rolled, at a temperature of' about 650 C. to a thickness of 0.50inch.

The one-half inch plate is then cold rolled to a thickness of 0.125 inch and annealed at 1000 C. for one hour. The annealed material'is' further cold rolledto a thickness of 0.010 inch after which the strip is annealed at 750 C. After one hour at the annealing tempera' ture, the strip is cooled under argon and is then placed in a furnace at 1200 C. and is maintained at that temperature for two hours to result in a secondary recrystallization structure exhibiting large grains. g

From the secondary recrystallization step, the strip is further cold rolled to a thickness of about (L003 inch.

The resulting tape is given a primary recrystallization: anneal at 750 C. for about five hours after which its temperature is raised to 950C. at a rate of about 100 C. per hour. The tape is maintained at 950 C. for about eight hours.

Typical properties of 5.5 percent Al-Fe tape made according to the foregoing example, and obtained from a D. C. hysteresis loop on the material, are as follows:

Table I B, (gauss) -s 9,200 B 16,900 1 9,400 H 3 0.45 Resistivity (microhm-cm'.) 65

The ingot thus produced is rolled at a-tempera-.-

illustration and not by way of limitais advantageous to have a material that exhibits a low remanence.

EXAMPLEII An iron-aluminum strip of'0.010 inch thickness is produced by a procedure exactly as used in Example I to produce the 0.010 inch strip. The strip is then subjected to a heat treatment identical to that applied to the 0.010 strip of Example I.- The strip is then cold rolled to a tape of a thickness of three mils. The tape is placed in a furnace at 750 C. and thetemperature raised at a rate of about 40 per hour to 1100 C. The tape is held at 1100 C. for'threehours after which it, is cooled in a coolingchamber under argon-to room temperature.

Typical properties for tapes produced according to the steps ofExample Hare asfollows:

Table II ag gauss .t a gauss. at 6000 gauss AH at 8000 gauss.

at; 10,000 gauss [-I=10 1.020 These data show that the product is characterized by the low remanence of 5,500'," furthermore, the product is obtained without simultaneously causing a deleterious dropinthe' induction at fields of 2 to 10 oersteds. These. tapes obviously can be used for suchapplications as pulse orcur'rent transformers, relays, meters and various Al-C. instruments. i

the permeabilities of the materials and obviously involve additional fabricating costs and problems. :Incontrast thereto, materials exhibiting low remanence are obtained by virtue of the present invention without. need for artificialities and without encountering the costs and problerns c'haracteristic of the prior art.

"It has been pointed out hereinbefore that properties. of the "resulting material are dependent upon the particu; lar treatment used. Within the broad ranges of cold reductionsand temperatures and times of heatjtreatment there 'arefn ore narrow ranges that are usedto advantage for developing a material for a predetermined enduse. By waver illustration, in producing materials in accordance witheachof Examples I and II, suitable intermed ate n s .dfi... u o that may b k u n ac Q h cold rollingoperations are as followsz-first cold roll, 70 to percent; second cold roll, to .95 percent-{third cold roll, .60 to.80 percent. Suitable ranges for -the heat treating-steps are as follows: annealing afterthefirst cold roll, 950 to 1050 C. for one-half to two hours heat treatment following the second cold roll, annealingat: 725 to'800 C. for 30 minutes to two hours followed by secondary recrystallization, afterfirst being cooled at.

1150 to 1250 C. for one-half to four hours. The proccdures of Examples I and II are difierent in, the. heat treatment used after' the third cold rolling operation. where tapes are tobe produced in accordance. with Example I, the tape, after the third coldreductiomfmay be. given aprirnary recrystallization anneal at 725 to 775 C;:for

about three to seven-hours, after whichhits temperature is raised at about 75 to per hour to about 900 to 950 C. and is held at that temperature for five to twelve.

hours. On the other hand, to-producea low 'remane'nce product as in Example II, the -tape,a-fter thelthirdwcold reductiomis placed inafurnace at a temperatureof'about 72 5,-to 775 C. and heated to'artemperaturc offaboufl To demonstrate the effect on the propertiesproduced by-changes in a given step, four tapes of 3 mil thickness each were produced by an identical procedure up to the heat treatment used after the third cold reduction. Each tape was then subjected to the same primary anneal. The properties of one of the tapes werethen measured. The other three tapes were each subjected to a difierent additional heat treatment. The details of the heat treatments and of the properties of the resulting materials. are summarized in the following table:

Table III 1 Reannealed Reanneeled by heating by heating 1 Treatment 750 C. from 750 to from 750 to Reanneeled iorli 950 C. at a 1,100 C. at a by heating hours rise of 100 rise of 100 for 5 hours a per hour. per hour. at 1,200 C.

Held 3 hours Held 3 hours at 950 C. at 1,100 C.

From these data it is evident that the properties may be affected by the heat treatment used. The best results from the pulse permeability standpoint are obtained with treatment 3 since a high results alongwith a low remanence and high "induction at 10 oersteds. The procedures" of-1 and 2 result in highsaturations albeit with a high remanence and thus are of importance where a high saturation is needed and a high remanenceis not deleterious. And finally, procedure i results inlowremanence though the saturation valuehas, in part, been sacrificed. It is apparent, that for the best remanence value, procedure 3 or 4 would be chosen; procedure 4 being preferred if the saturation value is not critical, and 3 being better if it is.

To this point, the invention has been described with respect to the use of a schedule of three cold rolling operations each being followed by a heat treatment. It is to be understood that those are, as presently advised, thought to be the best embodiments of the invention. However, the materials resulting, by way of example, at the point just prior to the third cold reduction also are novel and useful materials for magnetic applications. As evidence thereof, in the following table I have accumulated data on the properties of the 13 mil strip that was produced according to the following schedule: a 1: inch hot rolled ,plate of Fe-S.5 percent Al is cold reduced to a thickness of 0.125 inch and then annealed at 1000 C. for one hour. The strip is then cold rolled to 13 mils, annealed a 750 C. for one hour and then cooled. The cooled strip is then heated at 1200 C. for

coercive force. Moreover, the pulse permeability also is very satisfactory. Accordingly the 13 .mil product can be used where those properties are desired and the saturation induction at 10 oersteds is not of controlling significance.

From the foregoing it can be seen that this invention provides a new class of materials that may be used for magnetic applications. The high electrical resistivity of the iron-aluminumgives them an advantage over the corresponding iron-silicon or iron-nickel alloys in that lower core losses may be obtained. This high resistivity is of special interest in higher frequency applications.

such as 400 cycles and above. Those products exhibiting a low, remanence are superior to the Fe-Si and Fe-Ni products because with my Fe-Al products the actual permeability of .the material is retained inservice condi tions. .The mechanical properties of the Fe-Alytapes also commend their commercial adoption. For example, these materials are ductile and can be cold rolled to thin gaugeswithout diificultyi. As they are not highly strain sensitive, as'compared with Fe-Ni alloys, they are less susceptible to property. degradation through handling.

The products also are of lower unit Weight than Fe-Si,

or Fe-Ni materials, and hence may be of particular interest for airborne applications.

, Still another outstanding advantage of these new 'Fe-Al products is the fact that they maybe insulated by a simple low temperature air anneal, for example by heating the tape at 200 to 400 C, for about 30 minutes.

Such treatment causes an insulating A1 0 coating to form on thesurface; the coating is ductile and is essentially unaffected by subsequent handling operations. The coating is less than 10,- 'inch.in thickness. In providing Fe-Si and Fe-Ni materials with insulation, it has been accomplished by a procedure involving a scrupulous cleaning of the metal followed by applying the coating and then baking it. That necessitates additional expense, careful handling to avoid destroying the coating, and a loss in use of available space in core manufacture with those materials. Accordingly, the advantage in my i vention is evident. 0

According to the provisions of the patent statutes] have explained the principle ofmy invention and have described what I now' believe to represent its beste'ma bodiment. However, I desire to haveit understood that the invention may be practiced otherwise than as specifitwo hours to develop a large grain structure. The properties of the resulting material are:

Table IV B1 13,000 I B. 3,600 0.18 at 2000 gauss. 4, 200 at 4000 gauss.- 3, 320 91 at 6000 gauss 3,010 H at 8000 gauss r 2, 010 at 10,000 gauss 1, 800 11:10 1,000

cally described.

I claim as my invention:

1. A recrystallized iron-aluminum strip for magnetic applications which consists essentially of a ductile ironaluminum alloy consisting essentially of 3 to 7 weight percent of aluminum and the balance iron, said strip being about 1 to 18 mils thick and having the following properties as determined from a D. C. hysteresis loop on the strip: remanence not exceeding 9,500 gauss, an induction at 10 oersteds of at least 12,000 gauss and a coercive force not exceeding 0.50 oersted.

2. A recrystallized, doubly oriented iron-aluminum tape for magnetic applications which consists essentially of a ductile iron-aluminum alloy consistingessentially of 3 to 7 percent by weight of aluminum and the balance iron, said strip being about 1 to 6 mils thick and having the following properties as determined from a D. C. hysteresis loop on the tape: remanence not exceeding 5,500 gauss, an induction at 10 oersteds of at least 13,500 gauss, and a coercive force not exceeding 05 oersted.

3. A recrystallized, doubly oriented iron-aluminum tape for magnetic applications which consists essentially of a ductile iron-aluminum alloy consisting essentially of 3 to 7 weight percent of aluminum and the balance iron, said strip being about 1 to 6 mils thick and having the following properties as determined from a D. C. hysteresis loop on the tape: an induction at 10 oersteds of at least 14,500 gauss, and a coercive force not exceeding 0.45 oersted 4. That method of producing a doubly oriented ironaimnmujm' 's't'rip' for magnetic applications which "comprises cold rolling a hot rolled 'ductileiron-aluminum plate which consists essentially ofd' to' 7 weightprcent of aluininum'and the balance iron to reduce said plate by 60 to 85 percent of its thickness, heating the'resulting strip at above the recrystallization temperature to 7 stress relieve it, cold; rolling the stress-relieved strip toreduce it by 80-to 97 percent of its thickness, and then heating the resulting strip at a temperature and for a time sufficient to develop a large grain structure.

5. That method of producing a doubly oriented ironaluminum tape which comprises cold rolling a hot rolled ductile ironfalur'ninumplate of a thickness ofabout 0.4-to'0;65 -inch which consistsessentiallyof 3' to 7 weight percent "aluminumand the balance iron to reducesaid plate -by"6( )'to '85 percent of its thickness, stress relieving'the resulting reduced--strip, then-cold rolling the stress-relieved stripto reduce it by 80 to 97 percent of itsthickness, then heating the strip at an elevated temperature above its recrystallization temperature to cause grain'growth, coldrollingthe resulting strip to reduce it-by 49 to90' percent "of its thickness, and then heating the resultingtape at a temperature above its recrystallization temperature to develop cube-on-face orientation in the resulting grain structure.

.6; That method offlproducing doubly oriented. ironaluminurn tapes "which .comprisesQcold rolling. a2. hot rolled .plate of a thickness of about 0.4 to .0.65 :inch;and whichi 'consistsl essentially of about Ste 7. percent/by weight .oflaluminuin. and .thezbalance substantially .all irdnQby 70*to 80 percent of its thickness;.annealing the resulting strip at. a temperature of 8503to 1100 .C. to stress relieve. saidvstrip cold rolling the annealed-strip. to. reduce it' by 85m 95 .percent of. itsthickness, annealing. the strip at700 to 850. C., cooling.the strip then heating itat aternperatureof 1050". Cato ISOOO'C'. for

one-half toftwel've 'houtsto. cause grain growth, cold rolling the strip to reduce it 'by 60 .to -80 percent of its thickness, .andithen heating: the resulting. tape .at. a .temperature above its recrystallization temperature. to cause a cube-on-face orientation of the resulting grain structure. J t

'-'7. That method of producing ironvalumi'num .tapes whichv comprises cold rolling. a. hot rolled plate of a 3 thickness of about 0.4 to 0.65 inch and which consists. essentially of about'3 to 7 percent by'weightof'alnini num and'thebalance substantially all iron, by 70 t'dBO percent of its thickness, annealing the resulting strip at a temperature of 850 to 1100". C. to stress relieve said stripflcoldiijrplling the annealed stripfto reduce" it by: 85; to 9 5jpercentbf its thickness, annealing,thej'stripat 700. to 850?.Q, then heating it at a temperature'fqf 1050 .C. to 1300 C. for one-half to. twelve hourslto cause graingro'wth, cold rolling the strip to reduce it,

' by -to 80 percent of its thickness, heating the tape at a temperature of700. C. to 850 C. for about Ito 10 hours,'then raisingfthe temperature of the tape'at a rate of about? C.to 150 per hour to. a temperature of 900 to 1250 C. and maintaining that tempera ture for 5 to 12 hours.-

8. That. method of producing iron-aluminum tapes for magnetic applications which comprises .cold rolling a hot rolled plate of a thickness of about 0.4 to 0.65 inch and which consists essentially of about 3 to 7 percent by'weight of aluminum and the balance substantially all iron, by to percent of its thickness, annealing the resulting strip 'at a temperature of 950 to 1050 C. to stress relieve said strip, cold rolling the annealed strip to reduce it by m percent of its thickness,

annealing thestrip at 725 to 800 C., cooling the strip,

then heating it at a temperature of 1150 to 1250- C.

for one-half'to four hours to cause grain growth, cold;

rolling the strip to reduce it by 60 to 80 percent of its thickness, then heating the resulting tape at a temperature of: from 725 i C. .to 775 C. for 5 to 7thours,.then raising thetemperature from the temperature of about 725 to 7:75": C.- at a rate of 25 to 50 C. per hour to a higher temperature of about 1050;to. .1150. C. and maintaining said higher temperaturefor about two to four hours.

" Co. Inc.-, pages '210 to 220, especially pages 2l8't0 2Z0; 

1. A RECRYSTALLIZED IRON-ALUMINUM STRIP FOR MAGNETIC APPLICATIONS WHICH CONSISTS ESSENTIALLY OF A DUCTILE IRONALUMINUM ALLOY CONSISTING ESSENTIALLY OF 3 TO 7 WEIGHT PERCENT TO ALUMINUM AND THE BALANCE IRON, SAID STRIP BEING ABOUT 1 TO 18 MILS THICK AND AND HAVING THE FOLLOWING PROPERTIES AS DETERMINED FROM A D. C. HYSTERESIS LOOP ON THE STRIP: REMANENCE NOT EXCEEDING 9,500 GAUSS, AN INDUCTION AT 10 OERSTEDS OF LEAST 12,000 GAUSS, AN INDUCTION FORCE NOT EXCEEDING 0.50 OERSTED.
 6. THAT METHOD OF PRODUCING DOUBLY ORIENTED IRONALUMINUM TAPES WHICH COMPRISES COLD ROLLING A HOT ROLLED PLATE OF A THICKNESS OF ABOUT 0.4 TO 0.65 INCH AND WHICH CONSISTS ESSENTIALLY OF ABOUT 3 TO 7 PERCENT BY WEIGHT OF ALUMINUM AND THE BALANCE SUBSTANTIALLY ALL IRON, BY 70 TO 80 PERCENT OF ITS THICKNESS ANNEALING THE RESULTING STRIP AT A TEMPERATURE OF 850* TO 1100*C. TO STRESS RELIEVE SAID STRIP, COLD ROLLING THE ANNEALED STRIP TO REDUCE IT BY 85 TO 9K PERCENT OF ITS THICKNESS, ANNEALING THE STRIP AT 700* TO 850*C. COOLING THE STRIP, THEN HEATING IT AT A TEMPERATURE OF 1050*C. TO 1300*C. FOR ONE-HALF TO TWELVE HOURS TO CAUSE GRAIN GROWTH, COLD ROLLING THE STRIP TO REDUCE IT BY 60 TO 80 PERCENT OF ITS THICKNESS, AND THEN HEATING THE RESULTING TAPE AT A TEMPERATURE ABOVE ITS RECRYSTALLIZATION TEMPERATURE TO CAUSE A CUBE-ON-FACE ORIENTATION OF THE RESULTING GRAIN STRUCTURE. 